TERMINOLOGY. . 175. 



lisation, therefore, will form the most interesting subject of 

 our present inquiries. 



The different qualities of even faces consist in their 

 being either smooth, without elevations or depressions ; or in 

 their being provided with certain elevations and depressions, 

 which, however, are so faint, that the general appearance 

 of evenness and continuity of the faces is not affected by 

 their occurrence. Smooth faces also are called perfect, 

 particularly if they refer to cleavage ; and cleavage is said 

 to be the more perfect, the more its faces possess this 

 property. This is at the same time the reason why smooth 

 faces of cleavage are commonly much more easily obtain- 

 ed, than such as shew opposite properties. 



The quality of irregular faces likewise depends upon the 

 greater or less degree of smoothness of its inequalities ; and 

 according to this measure in particular, the conchoidal frac- 

 ture is said to be more or less perfect. The insensible dimi- 

 nution of this perfect smoothness produces the passage from 

 the conchoidal fracture into the uneven fracture (. 172.)- 



The intensity or the degree of lustre of the faces, is 

 proportional to the degree of their perfection. 



Those faces which are not smooth, may be striated, or 

 rough, or drusy. The most remarkable of these are the 

 striated faces. 



. 175. STRIATED FACES OF CRYSTALLISATION. 



The Striae upon the faces of crystallisation are 

 produced by the alternating re-appearance of the 

 faces of those simple forms, which are contained in 

 a compound one, and they are always parallel to 

 edges of combination. 



One of the forms most commonly occurring in rhombo- 

 liedral Quartz, is the combination of the regular six-sided 

 prism, with an isosceles six-sided pyramid, P. P 4- . The 

 faces of the prism are streaked horizontally, if the forms 

 be brought into the upright position. 



